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What are Megabacteriosis?

This common disease in your bird only lives in the gastrointestinal tract, and can be subclinical which means it can be present but in low levels and with no visible symptoms. Birds without symptoms may be responsible as carriers, spreading the disease. The organism is an extremely large gram positive rod shape. Megabacteria or Avian Gastric Yeast (AGY) can, in large numbers, cause severe disease to numerous bird species. Budgies are the most often affected, but it has also been found in parrotlets, lovebirds, king parrots, lorikeets, cockatiels, macaws, finches, cockatoos, ringneck parakeets, canaries and quail.

Once thought to be a mega-sized bacteria, it was later found to be of fungal origin. It has now been renamed as Avian Gastric Yeast.

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Symptoms of Megabacteriosis in Birds

  • Your bird may become unable to maintain its blood pressure and their feet become pale and cold to the touch due to internal ulceration
  • Sometimes an affected bird will look as though it is eating, but is actually just grinding the seed in its beak without ingesting it
  • Vomiting or regurgitation is common in affected flocks of budgies 
  • Death after long term weight loss if your bird is untreated 
  • Your bird may become quiet and fluffed (depressed)
  • Bulky dark feces 
  • Chronic weight loss 
  • Diarrhea 


Megabacteria causes two types of disease syndromes. 

Acute Disease

Many birds may get ill and die suddenly, usually within 12 to 24 hours. Your bird will appear quiet and very depressed. They may regurgitate a fluid that has traces of blood in it. 

Chronic Disease

This is more common and birds exhibit excessive weight loss.  The birds may exhibit gaping as they stretch their necks to try to swallow. A clear to white slime from regurgitation can coat the feathers and head, with signs of blood present. Watery droppings or diarrhea often shows undigested seed.

Causes of Megabacteriosis in Birds

  • Transmission of this disease is by infected feces contaminated water and is also thought that courtship feeding practices spread the disease as it has been found in the digestive tract 
  • In large numbers, the organism has the potential to ulcerate the lining of the glandular stomach which leads to loss of blood and tissue fluid 
  • The disease then progresses to progressive weight loss as the ulcers deepen which causes your bird to become anemic
  • Megabacteria also interfere with the digestion by decreasing the gastric acid formation
  • The result is an increase in PH enzyme activity that affects the koilin, causing it to become soft and thin 
  • The koilin (which is normally hard) in the gizzard of your bird is important for the grinding of food particles

Diagnosis of Megabacteriosis in Birds

This is a difficult disease to diagnose. Radiology is the most effective method, and using a contrasting dye such as barium can be helpful to isolate the enlarged and dilated glandular stomach. Areas that don’t contrast in the test may be due to the buildup of mucus that the organism induces. Other tests or practices your avian veterinarian specialist may perform is a crop wash, crop smears or even trying a proventricular wash to help show the present of megabacteriosis. One other way is to use a microscope to check fresh droppings or fecal smears for signs of the organism. 

Because the organism is so larger than other types of organisms, it is easy to see once under a microscope. Finding the bacteria or fungus does not necessarily mean it needs to be treated. Although it is a disease, it is often a secondary agent and only becomes a problem after becoming involved in some other disease. Poor management of your bird can lead to its immune system becoming weak, allowing the opportunity for the megabacteria/fungus to flourish. Low grade cases or exposure tends to encourage a natural immunity to this disease.

Treatment of Megabacteriosis in Birds

With ongoing research regarding the treatment of megabacteria/fungus there is no one definitive treatment yet that provides a one hundred percent cure. Traditional antibacterial drugs have shown limited success, with Amphotericin B, which is a polyene macrolide antifungal drug that is administered orally, found to be more effective. Unfortunately, treatment with Amphotericin B is complicated, costly and is not one hundred percent effective. Antimicrobial drug trials are underway at Texas A&M University where one drug, Fluconazole, has shown promise. 

Most testing has been done over a timespan of four weeks, with most researchers now saying it may be more effective over a longer time frame. The opinion of researchers also states that if your bird is showing no symptoms of the disease other than low fecal smear count in an otherwise healthy bird, treatment would be best suited to monitoring your bird’s health in case of any clinical changes. Testing and treating suspect birds with some anti-fungal product, along with good sanitation and quarantine practices, is the most successful method to manage this disease.

Recovery of Megabacteriosis in Birds

Because of the difficulty in treating a bird with this disease, the best course of action is preventing it being introduced into your aviary. The way to do this is to use repeated examinations of fecal wet-mounts. This will enable you to see changes in the health of your bird. It is important to practice good sanitation to prevent or eradicate the organism. Keeping water systems and the feeding bowls sanitised and without fecal contamination is vital. When you are introducing new birds to your flock, screening them first to see if the organism is present before adding them to the flock is best. This way, if they are infected, they can be kept isolated and you can treat them individually which will protect your whole flock from this disease.

Megabacteriosis Questions and Advice from Veterinary Professionals

Boy George
Ten Months
Mild condition
0 found helpful
Mild condition

Has Symptoms


My new budgie has completed 6 weeks in quarantine. He was thoroughly avian vet checked today with a diagnosis of Megabacteria. He was sent home with fourteen doses of anti-fungicide to add to his water daily.

He is ten months old. Will he ever be able to join my other three budgies? What can I do to help him? Did I cause this infection?

Dr. Michele King, DVM
Dr. Michele King, DVM
1611 Recommendations
Once Boy George has had all of his medication, he should be able to join the other birds, but it would be best to check with his veterinarian to make sure, since I do not know the details of his situation. I hope that he does well.

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5 Years
Fair condition
0 found helpful
Fair condition

What are your views on possibly acidifying the drinking water as a means to control or reduce the yeast?

"With stress, this innate protective mechanism can be lost. It is thought that by lowering (ie acidifying) gastric pH, an intestinal and stomach environment is created that not only makes it difficult for Megabacteria to establish in uninfected birds but also makes it difficult for Megabacteria to multiply in birds that are already infected. Usually, either citric acid (a white crystalline powder) at a dose of 1 teaspoon (3 g) to 4.5 – 6 litres or apple cider vinegar (acetic acid) 5 – 10 ml to 1 litre, are used. Being natural nutrients, at the above doses, there is no risk of a toxic reaction and so they can be used fairly freely. Their use also helps ensure that droppings passed build up an acidic dressing in the aviary, helping to inhibit Megabacterial survival in the environment."

And the BSAVE manual on psittacines says lack of grit has been implicated in megabacteriosis in budgies. I know that the use of grit it controversial and I have just decided, after many years, to reintroduce it, but rationed as only a dozen or less grits available in a small glass bowl (for 2 birds), with carefully controlled replenishment. What are your views on grit?

Many thanks


[email protected]

Dr. Callum Turner, DVM
Dr. Callum Turner, DVM
3320 Recommendations
Megabacteriosis is a little known yeast infection which was still classed as a bacterial infection until a short while ago; treatment of this condition is generally considered to be an antifungal medication and acidification of the water with a weak organic acid. I have no direct experience with Megabacteriosis, but I would suggest consulting with an Avian Specialist to discuss this with them; on the subject of whether or not to feed grit, is a controversial topic but I would be leaning towards not or in limited quantities. Regards Dr Callum Turner DVM

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